The present invention relates to a room temperature aged polymer film which has good hot tack, a low heat sealing temperature and high clarity. This invention addresses the unique problems associated with the film of such a polymer blend which has been room temperature aged. Polybutylene/polypropylene blends as oriented polypropylene (OPP) film heat sealing layers are known to lower the heat sealing temperature and give good hot tack strength during packaging. However, the room temperature aging of coated film normally results in an undesired increase in heat sealing temperature and deterioration of hot tack strength.
Hot tack is defined as the bonding strength between two pieces of polymer film at the point immediately after their having been joined together by application of heat and while the polymer is in semi-moltened/solidifying state. Films are often made into packages which are sealed by applying heat. The seal must retain sufficient strength to contain the materials within the package. Not only does the weight of the material or product contribute to a tendency for seals to release, but in addition, the pressure of air used to assist in transport of the product may exert additional pressure against the seal.
In addition to a good hot tack, a low heat sealing temperature is desired so that the film may be processed at a low temperature.
U.S. Pat. No. 4,230,767 discloses a packaging material with heat seal packaging properties composed of blends of a propylene/ethylene copolymer with a variety of other propylene olefins and copolymers. Propylene ethylene copolymers in '767 have an ethylene content of as low as 0.5% by weight. '767 does not disclose a film which, after room temperature aging, exhibits good hot tack and a low heat sealing temperature.